Silicon dioxide etch process which protects metal

ABSTRACT

The present invention is directed to a novel etching process for a semiconductor material which inhibits corrosion of metal comprised of pretreating the material, preferably with a surfactant, and then exposing the material to a mixture comprising salt, a buffered oxide etch, and optionally a surfactant.

FIELD OF THE INVENTION

The invention relates to a process for etching glass or SiO₂ with metalpresent wherein the metal is not etched during the etching of the glassor SiO₂.

BACKGROUND OF THE INVENTION

It is customary in the art to etch glass (amorphous SiO₂ films) with anaqueous solution of an acid, for example hydrofluoric acid, bufferedwith, e.g., ammonium fluoride. In the semiconductor industry, it iscommonly necessary to etch semiconductor materials like glass formed ona metal substrate, such as aluminum, or to etch glass having metalconducting lines on a silicon wafer or die. The acid during the etchingprocess attacks and corrodes the metal.

It has been reported that the addition of an expensive diol alcohol to abuffered acid etching solution inhibits its attack upon the metal.Moreover, it has been reported that a suitable amount of diol alcohol,e.g., ethylene glycol, to obtain this result is somewhere on the orderof fifty percent of the etching solution. For example, it has beenstated in U.S. Pat. No. 4,040,897 that a typical formulation using adiol alcohol to inhibit attack upon the metal is comprised as follows:five parts of a buffered oxide etch ("BOE") comprising thirty-four partsby weight of a forty percent aqueous ammonium fluoride solution, fiveparts of a forty-nine percent aqueous hydrofluoric acid solution, fiveparts of ethylene glycol and three parts water. In the same patent, itis reported that the diol alcohol containing etchant solution is nearlysaturated and that it has a high viscosity which renders it difficult torinse off and/or to etch small openings in a photoresist. It is alsoreported that the diol alcohol mixture tends to corrode the metalintermittently.

U.S. Pat. No. 4,040,897 titled "Etchants for Glass Films on MetalSubstrates" discloses a method of etching a glass layer on a metalsubstrate comprising contacting the glass layer with a dilute aqueousetch solution for a sufficient time to etch through the glass layer. Theetch solution disclosed in U.S. Pat. No. 4,040,897 is comprised of abuffered hydrofluoric acid, sodium chloride, and a fluorocarbonsurfactant. However, it has been found that using the process disclosedin U.S. Pat. No. 4,040,897 does not completely protect the metal fromattack and thus corrosion. Indeed, it has been found that upon exposureto the etchant solution and before the surfactant can coat the metal,the metal is attacked by the acid etchant and corroded to some extent.In addition, it has been found that the above-discussed processes do notprotect the metal substrate from galvanic action which is frequentlyencountered in wet etching processes with semiconductor materials.

SUMMARY OF THE INVENTION

The invention is directed to an etching process for semiconductormaterials which protects metals from corrosion comprising: (1)pretreating the material to be etched (to protect the metal), and (2)exposing said material to a mixture of a buffered oxide etch and a salt.The salt is added in a sufficient amount so that the mixture maintains anatural saturation equilibrium during the etching process or anyelectrolysis effects are neutralized. Preferably, the material ispretreated with a surfactant. Alternatively, the process of the presentinvention may include pretreating only the metal with a surfactant. Inaddition, the exposing step may include a surfactant.

The invention is also directed to an etchant for semiconductor materialscomprising: (1) from about 0.03 to about 0.06 percent by weightsurfactant, (2) from about 15 to about 30 percent by weight bufferedoxide etch, and (3) a salt in sufficient quantity to neutralizeelectrolysis effects.

The invention is further directed to a semiconductor material etched bythe process of the present invention. The process comprises: (1)pretreating the material with a fluorocarbon surfactant, and (2)exposing said pretreated material to an etchant bath comprising fromabout 2 to about 4 percent by weight salt, from about 16 to about 28percent by weight buffered oxide etch, and (3) agitating the bath. Theetchant bath is preferably comprised of sodium chloride in an mount suchthat the bath maintains a natural saturation equilibrium. Optionally, ifdesired, the etchant bath may include a surfactant.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous etching solution of the present invention is directed to theetching of glass or SiO₂ wherein metal is present. More specifically,the etching process of the present invention is directed to a processfor etching glass wherein metal is present without attacking and/or thuscorroding the metal during the etching process. The etching solution ofthe present invention includes conventional buffered oxide etches("BOE"), a salt, and a surfactant. In addition to the water of the BOEsolution, more water may be added to the overall etching solution.

Conventional buffered oxide etch solutions may be employed in thepractice of the present invention. Conventional BOEs may be comprised ofvarying ratios of ammonium fluoride and hydrofluoric acid in an aqueoussolution. The preferred BOE is ammonium fluoride "BOE 410" manufacturedby Olin and Hunt which is comprised of ammonium fluoride, hydrofluoricacid and water in the following ratio: four parts of a 28.5 to 40percent ammonium fluoride solution, one part of a 7.0 to 8.4 percenthydrofluoric acid solution, and five parts water. In addition, a BOE 710may be utilized which is comprised of: seven parts of a 33 to 35 percentammonium fluoride solution, one part of a 6 to 9 percent hydrofluoricacid solution, and two parts water.

A preferable salt useful in the practice of the present invention issodium chloride. However, other alkali halide salts may be used in thepractice of the present invention. For example, potassium chloride,potassium bromide, sodium bromide, cesium chloride, cesium bromide,rubidium chloride and rubidium bromide may be utilized.

In order to help protect the metal from corrosion, it has been foundthat it is necessary to always keep the etching solution at a naturalsaturation equilibrium. It is believed this neutralizes the electrolysiseffects of the process. To achieve this, it has been found necessary toalways keep the salt in excess of its solubility limit in the solution.This can be ensured by adding the salt last during the preparation ofthe etchant bath. Preferably, the etching solution or bath includes anexcess amount of solid salt settled in the bottom of the etchingcontainer.

In the practice of the present invention, preferably a surfactant isemployed which is not detrimental to the etching process but, is howevercapable of protecting the metal substrate from attack by the acid in theetch. Surfactants useful in the practice of the present inventioninclude the fluorocarbon surfactants: "Fluorad FC-120", "Fluorad FC-129"and "Fluorad FC-135". Fluorad FC-120, FC-129, and FC-135 aremanufactured by the Specialty Chemicals Division of 3M. FC-120 iscomprised of: (1) 37.5 percent by weight 2-butoxyethanol, (2) 36.0-39.0percent by weight water, (3) 21.0-24.0 percent ammonium perfluoralkylsulfonate (C10), (4) 1.0-3.0 percent by weight ammonium perfluoralkylsulfonate (C8), and (5) 0.1-1.0 percent by weight diisopropyl ether.FC-129 comprises: (1) 40.0-44.0 percent by weight potassium fluoralkylcarboxylate (C8), (2) 32.0 percent by weight water, (3) 14.0 percent byweight 2-butoxyethanol, (4) 4.0 percent by weight ethanol, (5) 1.0-5.0percent by weight potassium fluoralkyl carboxylate (C6), (6) 1.0-5.0percent potassium fluoralkyl carboxylate (C4), (7) 1.0-3.0 percent byweight potassium fluoralkyl carboxylate (C7), and (8) 0.1-1.0 percent byweight potassium fluoralkyl carboxylate (C5). FC-135 comprises: (1) 40.0to 44.0 percent by weight fluoralkyl quaternary ammonium iodide (C8),(2) 33.0 percent by weight isopropyl alcohol, (3) 17.0 percent by weightwater, (4) 1.0-5.0 percent by weight fluoralkyl quaternary ammoniumiodide (C6), (5) 1.0-4.0 percent by weight fluoralkyl quaternaryammonium iodide (C7), (6) 1.0-4.0 percent by weight fluoralkylquaternary ammonium iodide (C4), (7) 1.0-2.0 percent by weightfluoralkyl quaternary ammonium iodide, and (8) 0.1 percent by weightomethyl iodide.

In the practice of the present invention, the following steps may befollowed: (1) the semiconductor material is pretreated, preferably byexposure to about 500 to 1000 ppm surfactant for approximately oneminute (however, other amounts of surfactant for varying amounts of timemay be utilized depending upon the particular surfactant used and theamount and type of metal to be protected), and (2) the semiconductormaterial is exposed to the etching solution for the necessary time toachieve the result desired, e.g., removal of oxide.

The etching solution or mixture is made by adding the followingconstituents into a container in the order listed: (1) from about 70weight percent to about 80 weight percent water, (2) from about 15weight percent to about 30 weight percent buffered oxide etch, and (3)from about 2 weight percent to about 4 weight percent salt. In addition,the etchant bath may include from about 0.03 to about 0.06 weightpercent surfactant. Preferably, the etchant, etching solution or mixtureis comprised of 0.05 percent by weight surfactant, 4.0 percent by weightsodium chloride, 76.95 percent by weight water, and 19.0 percent byweight BOE 410. It should be recognized that while it is preferred touse the same surfactant for the pretreatment step and the exposing stepit is not required. In fact, if pretreated appropriately, there is noneed for a surfactant in the exposure step of the process in certaincircumstances, e.g., when the etch time is very short.

In the normal etching process, that is, comprising an etching bath, thesemiconductor material is dipped into the bath for the necessary time toachieve the desired result under agitation. Upon completion of thedesired purpose, for example, the removal of oxide, the semiconductormaterial is preferably rinsed for approximately one minute with, forexample, deionized water.

In the preparation of the etching solution and/or bath as stated above,it has been found necessary to keep the salt, for example, sodiumchloride, in excess of its solubility limit in the solution. The excesssalt keeps the solution at a natural equilibrium which is believednecessary to neutralize the electrolysis effects of the etching processso that the metal is not attacked or corroded. It is important to notethat the mixing order of the bath is specific, as set forth above. Forinstance, if the salt is totally dissolved in the water prior to theaddition of the remaining constituents, then the metal will delaminatefrom the substrate instantaneously from areas where galvanic actionoccurs. It is believed this is due to the excess sodium and chlorideions in the solution. On the other hand, if the salt is left outcompletely, corrosion of the metal may occur in areas where the oppositegalvanic reaction occurs.

While the invention is acceptable to various modifications and alternateforms, specific embodiments have been shown by way of example and havebeen described in detail herein. However, it should be understood thatthe invention is not intended to be limited to particular embodimentsdisclosed. Rather, the invention is to cover all modifications,equivalents and alternatives falling within the spirit and the scope ofthe invention as defined by the appended claims.

We claim:
 1. A process for etching semiconductor material that has ametal component, comprising the steps of:treating the semiconductormaterial with a surfactant to protect the metal component from corrosionduring etching; forming an etching solution from a buffered oxide etchand a salt with the salt being of an amount in excess of a solubilitylimit for the etching solution; and disposing the treated semiconductormaterial in the etching solution to effect etching.
 2. The process ofclaim 1, wherein the surfactant includes a fluorocarbon surfactant. 3.The process of claim 1, wherein the salt includes sodium chloride. 4.The process of claim 1, wherein the buffered oxide etch further includesammonium fluoride, hydrofluoric acid, and water.
 5. A process foretching semiconductor material that has a glass component and a metalcomponent in an etching solution formed from a buffered oxide etch and asalt, the improvement comprising the steps of:treating the semiconductormaterial with a surfactant before disposing the semiconductor materialin the etching solution to protect the metal component from corrosionduring etching; forming the etching solution from a buffered oxide etchand a salt such that the salt is in an amount in excess of a solubilitylimit for the etching solution.
 6. The process of claim 5, wherein thesurfactant includes a fluorocarbon surfactant.
 7. A process for etchingsemiconductor material that has a metal component, comprising the stepsof:treating the semiconductor material with a first surfactant toprotect the metal component from corrosion during etching; forming anetching solution from a buffered oxide etch, a second surfactant, and asalt with the salt being of an amount in excess of a solubility limitfor the etching solution; and disposing the treated semiconductormaterial in the etching solution to effect etching.
 8. The process ofclaim 7, wherein the first surfactant includes a fluorocarbonsurfactant.
 9. The process of claim 7, wherein the second surfactantincludes a fluorocarbon surfactant.
 10. The process of claim 7, whereinthe salt includes sodium chloride.
 11. The process of claim 7, whereinthe buffered oxide etch includes ammonium fluoride, hydrofluoric acid,and water.